Career change or promotion
One of the most reasons for going to graduate school is because of a career change or to get a promotion. Higher positions in almost any company or organization will typically require an advanced degree or certificate of some sort. This is a great reason to go to graduate school, but you want to be sure the degree you are seeking is the one that will further your career.
To help with this, ask your superior at your job or the job you are applying to. Know exactly what your employer wants before jumping into a degree field that might not help you in the long run. Another good reason to speak with your employer first is because they may offer tuition reimbursement or extra incentives for completing a degree that they know will help improve the company.
Furthering your education to boost your expertise
Satisfying your intellectual urge to succeed is another good reason to go to graduate school, though most would say it shouldn't be your determining factor. Education is never a bad thing and it will always pay off somehow in the end. Whether it be for a better or higher paying job, to help you be better at your job, or simply because you like to learn, education will be beneficial in the long run.
Especially if you already have a career, going to graduate school is a great way to improve your skillset and become a more knowledgeable and effective professional. It will be a lot of work going to school and working full-time, but if you take a class or two a semester, it will be over before you know it and you'll have an advanced degree for the rest of your life.
You are in the financial situation to do so
Unfortunately, your financial situation might be the determining factor for whether or not you end up going to graduate school. Grad schools do offer financial aid like they do for an undergraduate degree, but like anything, you shouldn't just take out loans willy-nilly. Grad school typically costs substantially more than your undergraduate degree did, especially if it has been some time since you last went to school. Assess your own situation and determine if the end product will pay off.
Likewise, there are scholarships and opportunities to get your tuition paid for in full or at least in part. Many employers offer furthering education reimbursement and some will even cover the entire cost.
Also look for opportunities within the school, including teaching and research assistantships. Working while going to grad school is almost always a must, especially in the majority of people's current economic situation.
You know what you want to do
You need to know what you want to do before you start a graduate program. Grad school isn't like an undergraduate program where you have to take a bunch of general requirements and then get to work on your major. You will have to decide exactly what you want your degree in so that you can apply for that specific program and meet those specific requirements. Each program is different. Plus, you don't want to end up having to switch to a different program half way through. That's just a waste of time and money.
Going to graduate school can certainly be beneficial depending on your career goals and current financial situation. Working while going to grad school is becoming the new norm, so don't leave that out as an option either. Good luck in whatever you decide to do.